1. Field of the Invention
The present invention relates to a semiconductor device having a MIS transistor, a CCD device or the like, and its fabrication method.
2. Description of the Related Art
The conventional MIS transistor includes a protective device for draining the charges on a gate electrode separately from the MIS transistor. For example, a diode is used as the protective device.
The above MIS transistor and the protective device are almost concurrently formed on a semiconductor body. Thus, in a wiring process (for example, metal wiring process) after formation of the MIS transistor and the protective device, the gate electrode of the MIS transistor is connected to the protective device by means of a wiring.
Also, the conventional CCD device does not include a protective device for draining the charges on each transfer electrode thereof.
In the MIS transistor of the above structure, the gate electrode is connected to the protective device by a wiring during the wiring process. Accordingly, in the process prior to the wiring process, the charge is stored on the film surface forming the gate electrode or the surface of the gate electrode with a treatment using plasma and ions (for example, a plasma etching treatment, reactive ion etching treatment, or ion implantation treatment). As a result, the gate electrode is charged up to several tens to several hundreds of volt, so that the threshold voltage Vth varies. Consequently, the operational margin of the MIS transistor is made smaller, thereby degrading the performance of the transistor.
To cope with the above problem, in a treatment using plasma and ions, an antistatic ion has been added in the reactive gas and the etching gas. However, this technique has the following disadvantages: namely, by the addition of the antistatic ion, it is possible to reduce the amount of the electrical charges on the gate electrode of the MIS transistor during the treatment using plasma and ions, and it is impossible to reduce the amount of the electrical charges to such a level as to eliminate the variation in the threshold voltage Vth. This makes it difficult to prevent the degradation in the performance of the MIS transistor due to the variation in the threshold voltage Vth.
Also, in the above CCD device, since the protective device is not connected to each transfer electrode, the transfer electrode is charged by the treatment using plasma and ions in the process of forming the transfer electrode and the subsequent process. As a result, the surface potential of the transfer electrode is varied, which causes malfunctions in transfer of the signal charge, thereby degrading the reliability of the CCD device.